Conventional PTAT voltage generators employ a diode-based bandgap to provide a current that was independent of any process variation and had a positive temperature coefficient. This current was mirrored using P-type transistors into a resistor string, which yielded a voltage that had the same positive voltage coefficient as the current. Since the voltage across the diode and the resistance value of the resistor do not track during manufacturing there is a mismatch in the drain voltages between the two P-type transistors. Thus, the current is not properly mirrored and some error is introduced.
U.S. Pat. No. 6,900,689 to Kimura discloses a Complementary MOS (CMOS) reference voltage circuit, formed on a semiconductor integrated circuit that outputs a reference voltage having a temperature-independent characteristic. The circuit included a first and second diode-connected transistors (or diodes), respectively grounded and driven with two constant currents bearing a constant current ratio to each other. An amplifying unit which included a differential voltage of output voltages from the first and second transistors by a preset factor and for summing the amplified differential voltage to an output voltage of the first or second transistor. The amplifying and summing unit included two operational amplifiers (OTAs) and a current mirror circuit. The first OTA is fed with the differential voltage and the second OTA has a reverse phase input terminal fed with an output voltage from the first or second transistor and a forward phase input terminal connected to its output terminal and driven with a current proportional to an output current of the first OTA, with an output terminal voltage of the second OTA being used as an output voltage. Accordingly, the CMOS reference voltage circuit comprised a first and second diode-connected transistors (or diodes), which are grounded, and are driven respectively by two constant currents, bearing a constant current ratio, and means for amplifying a differential voltage between output voltages of the first and second diode-connected transistors (or diodes) by a predetermined constant factor and summing the resulting amplified voltage to an output voltage of the first or second diode-connected transistor (or diode), in which said means for amplification and summation includes first and second operational transconductance amplifiers (OTAs) and a current mirror circuit, in which the first OTA is fed with the differential voltage, the second OTA has a first input terminal(−) fed with an output voltage from the first or second diode-connected transistor (or diode) and a second input terminal(+) connected to an output terminal and driven with a current proportional to the output current of said first OTA, an output terminal voltage of the second OTA being an output reference voltage.
U.S. Pat. No. 6,323,628 to Park disclosed a voltage regulator that establishes a bandgap voltage reference and achieves output voltage regulation with a single feedback loop. The bandgap voltage reference is established by equal current flow through each of two branches of a proportional to absolute temperature current mirror. The equal current flow through the two branches of the proportional to absolute temperature current mirror is achieved by the feedback loop controlling the current flow in response to the bandgap voltage reference. This same feedback loop, responsible for establishing the bandgap voltage, also establishes the regulated output voltage through a pass transistor by means of maintaining a fixed voltage ratio between the bandgap voltage and the regulated output voltage through a resistor string . . . Accordingly, the voltage regulator included a proportional to absolute temperature current mirror having first and second current branches for establishing a bandgap voltage when current flow through the first and second current branches is equal and a resistor string coupled to the proportional to absolute temperature current mirror and responsive to the bandgap voltage for developing a regulated voltage from the bandgap voltage that is supplied to a load. Also included in this voltage regulator are output means between the proportional to absolute temperature current mirror and the resistor string for supplying output current to the load while maintaining the regulated voltage constant and an inverting gain stage coupled to the proportional to absolute temperature current mirror for sensing relative current flow through the first and second current branches in the proportional to absolute temperature current mirror and for controlling the output means to maintain the regulated voltage constant.
The conventional prior art PTAT circuits introduce errors because the current was not properly mirrored.